In some mobile communication systems including Long Term Evolution (LTE), uplink synchronization establishment between a terminal and a wireless base station apparatus is performed using a channel that a certain user is allowed to use, which is called a random access channel (RACH). A series of procedures for establishing such uplink synchronization is called a RACH procedure, and is obtained, for example, by the following procedure in the LTE.
A mobile communication terminal transmits a RACH preamble to a wireless base station apparatus using a physical random access channel (PRACH). A time period in which the transmission of the RACH preamble through the PRACH is performed and the range of wireless resources are notified, in advance, to the mobile communication terminal using broadcast information from the wireless base station apparatus. However, it is difficult for the wireless base station apparatus to recognize a resource used for the transmission through the PRACH from among the notified wireless resources and timing at which the resource is used in the notified time period. Therefore, the wireless base station apparatus determines the presence or absence of transmission through the PRACH in each of the wireless resources by performing reception for all of the wireless resources of the specified range in the specified time period. When the wireless base station apparatus determines the presence of the transmission, the wireless base station apparatus secures a wireless resource of a RACH Message 3, and transmits information on the wireless resource to the mobile communication terminal using a RACH response for the PRACH resource. The mobile communication terminal tries to perform reception of the transmitted RACH response for the PRACH resource, and transmits the RACH Message 3 using the wireless resource that has been notified using the RACH response when the reception has been performed successfully. In the RACH Message 3, a connection request and the like from the mobile communication terminal are stored. The mobile communication terminal assigns a unique ID that identifies the terminal of the self to the RACH Message 3 for the first time. The wireless base station apparatus tries to perform reception of the RACH Message 3 and when the reception has been performed successfully, the wireless base station apparatus notifies the mobile communication terminal of the ID stored in the RACH Message 3 using a RACH Message 4. The mobile communication terminal receives the RACH Message 4, and confirms the stored ID. When the ID corresponds to the mobile communication terminal of the self, the mobile communication terminal determines that uplink synchronization establishment has been performed successfully, and ends the RACH procedure. On the contrary, when the stored ID does not correspond to the mobile communication terminal of the self, the mobile communication terminal performs the RACH procedure again.
Here, it is difficult to accurately determine the presence or absence of transmission through the PRACH from among channels used for the RACH procedure in the wireless base station apparatus because the transmission through the PRACH depends on a wireless environment such as noise or a transmission path, or there is no information indicating when the user uses the PRACH for transmission. Therefore, there is possibility that the wireless base station apparatus fails to detect transmission through the PRACH even when the transmission through the PRACH has been performed (missed detection), or that the wireless base station apparatus detects transmission through the PRACH by mistake even when the transmission through the PRACH has not been performed (false alarm). In addition, the case in which the wireless base station apparatus fails to detect transmission through the PRACH even when the transmission through the PRACH has been actually performed may be referred to as “missed detection”. In addition, the case in which false alarm occurs when the transmission through the PRACH is not performed may be referred to as “false alarm”. In addition, in the following description, “missed detection” and “false alarm” may be collectively simply referred to as “detection error”.
Specifically when the false alarm has occurred, the wireless base station apparatus performs allocation of a wireless resource of the RACH Message 3, and transmission of the RACH response, as a result of the false alarm for the transmission through the PRACH resource, which has not been performed. However, in this case, there is no mobile communication terminal that performs transmission of the RACH Message 3, so that the secured wireless resource is not used and become useless. In addition, a physical channel of the RACH Message 3 is a physical upiink shared channel (PUSCH) through which retransmission is performed, so that the wireless base station apparatus keeps a wireless resource for the retransmission until the number of times of retransmission reaches the maximum limit.
When such a useless wireless resource exists, throughputs of the entire cell are reduced, and a reduction in the accommodation efficiency is caused eventually. It is desirable that a threshold value of PRACH detection is determined appropriately to avoid such a reduction in the performance.
The wireless base station apparatus typically determines the presence or absence of the RACH preamble transmitted through a PRACH by comparing a reception signal to interference ratio (SIR) or some characteristic relating to reception power and a threshold value that has been set in advance. As the threshold value used for the detection becomes larger, a possibility becomes smaller that false alarm for the RACH preamble transmitted through the PRACH occurs in the wireless base station apparatus, but a possibility becomes larger that the wireless base station apparatus fails to detect the transmission through the PRACH that has been actually performed. On the contrary, as the threshold value becomes smaller, a possibility may become smaller that the wireless base station apparatus fails to detect the transmission through the PRACH that has been actually transmitted, but a possibility becomes larger that false alarm for the RACH preamble transmitted through the PRACH occurs. A trade-off relationship is established between a reduction in the false alarm and a reduction in the missed detection, and it is desirable that the threshold value is determined so that balance between the reduction in the false alarm and the reduction in the missed detection is maintained without bias.
For example, as a technology by which a detection threshold value of a preamble is determined, there is a technology in a related art by which a probability of false alarm of the preamble is calculated from the presence or absence of the preamble in a message that has been received from a mobile communication terminal, and a threshold value is determined from the calculated probability of the false alarm.
Japanese Laid-open Patent Publication No. 2011-114716 is the related art.